The present invention relates to computer systems which are configured to receive expansion cards.
Generally in computer systems and especially in personal computer systems, data are transferred between various elements such as a central processing unit (CPU), input/output (I/O) adapters, I/O devices such as bus masters (i.e., elements which can control the computer system) or bus slaves as well as memory devices such as the system memory. These elements are often interconnected via a system bus which is part of a system architecture. The architecture is designed for the movement of data, address and command information with or between these elements. In personal computer systems, one such architecture has become an industry standard and is here referred to as the Family I bus architecture.
The Family I bus architecture has become widely used by personal computers such as the IBM PC and PC/AT. The Family I bus architecture transfers information using eight parallel paths (an 8-bit wide bus) or 16 parallel paths (a 16-bit wide bus). A significant feature of the Family 1 bus architecture is the requirement of performing all transfers in synchronization with one basic clock signal, hereinafter referred to as a CLK signal. The CLK signal is an 8 MHZ signal which is provided to every element which is connected to the bus.
Because of the popularity of the Family I bus architecture, it has become advantageous to extend the Family I architecture to a 32-bit wide format. Customer acceptance requires maintaining downward compatibility with the original Family I bus architecture. Presently, maintaining downward compatibility requires that all elements on the architecture's bus operate at the original Family I clock rate of approximately 8 MHz. Techniques for allowing the bus to operate at different speeds have been proposed.